1. Field of the Invention (Technical Field)
The present invention relates to electric field probes for high-speed integrated circuits and semiconductor devices.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Operating speeds of electronic devices and integrated circuits (IC) are rapidly surpassing the capabilities of conventional electronic measurement instrumentation. Evaluation of fast semiconductor devices requires both high temporal resolution and the ability to probe internal points of an IC noninvasively. While optical methods promise ultrashort temporal resolution (xcx9c50 fs) and noninvasive probing of internal structures of ICs, conventional electro-optic sampling requires semiconductors without inversion symmetry such as GaAs and InP. Centrosymmetric semiconductors such as silicon and germanium require an external electro-optic probe placed within the fringe electric field flux lines of the region of interest, reducing sensitivity and adding parasitic capacitance to the probed circuit. Since silicon MOSFET technology is the dominate technology used in logic and memory devices, it is important to develop noninvasive techniques that work on both centrosymmetric silicon-based devices in addition to GaAs and InP devices.
Conventional electronic measurement systems are no longer adequate to measure waveforms in the fastest electronic devices and integrated circuits. Fortunately, the fastest electronic devices have tended to be III-IV based semiconductors that are not centrosymmetric. Because of the lack of inversion symmetry in these crystals, they exhibit the Pockels effect. When an electric field is applied to a substance that exhibits the Pockels effect, the polarization of a probe beam passing through the substance can be rotated. This rotation is proportional to the applied electric field, and is easily observed by either transmission through crossed polarizers or a phase sensitive interferometer. Techniques and devices have been developed that take advantage of the Pockels effect to probe electric fields in various semiconductors that are not centrosymmetric. These instruments do not function on centrosymmetric semiconductors such as silicon or germanium, however. With clock rates of commercial silicon-based devices approaching 1 GHz and beyond, new technologies are needed to probe these devices.
One approach to probe centrosymmetric semiconductors is to place a minute external probe, composed of material that exhibits the Pockels effect, very close to the device/region to be probed. J. M. Wiesenfield, IBM J. Res. Develop., 34:141 (1990). Unfortunately, such an approach cannot image or probe internal devices in the integrated circuit. Also, such approaches can add parasitic capacitance to the device being probed which will skew any measurements. The present invention comprises a general technique for probing semiconductor devices and integrated circuits that is useful with any semiconductor material. Since the method of the present invention does not use an external probe, it is adaptable to imaging and does not add any parasitic capacitance.
The present invention relies on DC electric field induced second harmonic generation (which we call Field Induced Second Harmonic generation, or FISH). Use of FISH for the applications described herein comprises a sensitive probe for a p-n junction of a semiconductor device. The present invention is applicable to semiconductor device research and to the development and testing of commercial integrated circuits.
Publications providing background to the present invention include: K. A. Peterson, et al., Opt. Lett., 26:438 (2001); K. A. Peterson, et al., OSA Nonlinear Optics, Materials, Fundamentals, and Applications Topical Meeting, Kaua""i, Hi. (August 2000); and D. J. Kane et al., OSA Annual Meeting, Providence, R.I. (October 2000).
The present invention is of a non-invasive electro-optic probe and probe method for probing electric fields in semiconductor devices comprising: directing laser output at a semiconductor device being probed employing a laser having an operating wavelength such that photon energy of a second harmonic wavelength of the operating wavelength is below a band gap of a semiconductor comprised by the semiconductor device being probed; and detecting second harmonic wavelength but not operating wavelength radiation from the semiconductor device being probed. In the preferred embodiment, the laser is a mode-locked laser. The detection apparatus comprises a photomultiplier tube, preferably with an interference filter to minimize two-photo absorption in a photocathode of the photomultiplier tube. Phase sensitive detection of the second harmonic wavelength is employed, preferably detecting the second harmonic wavelength at a frequency difference between an electric field modulation and a probe modulation, and preferably via a chopper between the laser and the material to be probed and a lock-in amplifier receiving output from the detection apparatus and providing input to the laser. The semiconductor device is preferably operated while it is being probed, and the electric field probed is preferably that present at a p-n junction of the semiconductor device being probed. The electric field probed is preferably modulated.
A primary object of the present invention is to use DC electric field induced second harmonic (FISH) generation to measure electric fields in electronic devices.
A primary advantage of the present invention is that it is robust in nature because of its inherent background free technique for probing integrated circuits.
Another advantage of the present invention is that it is sufficiently sensitive for the measurement of electric fields in semiconductors.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.